The present invention relates to an apparatus for delivering fluid substances in metered quantities, wherein metering is performed by weight control of the delivered substance. The invention is above all applicable in the preparation of dyeing agents used in printing or dye works although other applications are possible for which accurate controls and extreme precision of the delivered quantities of fluid substances having high viscosity, are required.
In preparing coloring or dyeing substances, particularly for dyeing works, fluid components are used as a basis for a dyeing substance which must be mixed together in metered quantities, strictly observing the required tolerances. Generally use is made of special delivering devices provided with motor-driven valves, to carry out weight control by modulating the flow rates of components and controlling the delivery times of the valves.
In general the flow rate may be modulated by appropriately varying the outflow from a valve, or by providing separate delivery valves having different flow rates; the valves are sequentially opened for predetermined lengths of time, under the control of a programmable process unit which receives control signals from a weighing device.
In weight metering systems which make use of flow modulating valves, wherein the weight is controlled by gradually reducing the flow of the substance to be metered, the final adjustment of the weight is the most delicate phase, in that it can be easily affected by many external factors, such as the viscosity of the fluids temperature and, mainly, pressure or hydraulic head acting on the delivery device.
In traditional apparatus, in which the substances are simply delivered by gravity, the delivery device is directly connected to a storage tank of adequate capacity, positioned at a predetermined height above the delivering device on a support structure. Since the flow rate delivered by the device is a direct function of the hydraulic head which acts on the delivery device itself, clearly in systems of this kind considerable variations in flow may occur, particularly in the final weight adjustment phase, given that the hydraulic head changes as the filling conditions of the storage tank vary.
In systems of this kind constant care and attention is required to prevent excessive lowering of the level of fluid in the storage tank, without nevertheless eliminating the inherent limitations and defects of these systems.
In place of normal gravity delivery systems, generally preferred due to their constructional simplicity, the use of reciprocating positive-displacement pumps has been proposed. However the use of positive-displacement pumps in combination with delivery devices having modulated flow rates, involves extremely complex management of the whole system, since the feed flow rate of the pumps must in turn be modulated correspondingly to the modulation of the flow rates of the delivery system. It is in fact clear that, if the flow rate of the feed pump remains constant, when the flow rates of the delivery heads vary, differences in pressure would arise which would introduce further causes for error. Moreover, the use of reciprocating positive-displacement pumps does not allow to have a constant hydraulic head and an absolutely constant flow rate, particularly when the reciprocating movement of the piston is reversed, since when the stroke is reversed, there is an interruption in the supply or in any case an irregularity in the feeding phase which may jeopardize or have a negative effect in the final phase of weight adjustment of delivered substance.
Therefore, even those systems which use positive-displacement pumps for feeding and delivering fluid substances, in addition to being extremely complex to handle and high in cost, do not solve the problem of control and of perfect weight metering of the delivered substances.
A typical example of prior known apparatus for automatically metering fluid substances for dye or printing works, which makes use of metering valves directly connected to respective storage tanks, or by pumping devices, is disclosed in EP-A-412 598 which constitute the nearest prior art.
EP-A-477 811 and U.S. Pat. No. 4,018,671 disclose pressure delivery systems in which a pressurized vessel holding a volume of the substance to be delivered, is connected to an air or gas supply and to a storage tank respectively. More particularly, EP-A 477 811 discloses a metering system for delivering liquid substance for use in applications such as liquid chromatography or similar applications in which the volumes of liquid being handled are quite small being of the order of micro liters, and in which the delivered liquid has a substantially low viscosity. Furthermore, according to said document the chromatography liquid solvent is metered under pressure and volume controlled conditions in an elongated vessel comprising upper or low level detectors for a known volume of the liquid; the time taken for the liquid to fall from the upper to a lower detected levels gives a measure of the rate of the delivered liquid; no weigh control or modulation of the delivery is suggested or performed in the apparatus. U.S. Pat. No. 4,018,671 discloses an intermitted catalyst addition system in the field of hydrocarbon conversion and related process or apparatus, in which an addition hopper is filled by gravity from a storage tank and utility air is supplied to the same hopper and to a catalyst carrier line, in air conveyor systems.
None of the above prior art suggests or makes obvious the use of a pressure-aided gravity delivering system in combination with a particular modulating valve delivering device particularly suitable for dye and printing works of extreme simplicity and having high control degree according to present invention.